It is known to place a circular preform of knitted metallic wire mesh in a compression die and to compress the preform longitudinally or radially, or both, to produce a shaped product. Such known methods frequently begin with the production of a mesh sock on a circular knitting machine. The sock might be cut, flattened and wound, perhaps together with other material such as graphite, into a spiral preform to provide the desired thickness of a final product. A description of an improved process of this type, and product made, including a detailed discussion of the compression step and compression die, may be found in commonly-owned copending U.S. patent application Ser. No. 07/661,282 now abandoned, filed Feb. 26, 1991. That application is hereby incorporated by reference.
In the past, it has been known to produce a slag filter for an automotive airbag by longitudinally compressing a single-ply sock of knitted wire mesh in a compression die. By "compressing a single-ply sock" is meant that the sock is not first flattened to produce a two-ply item, as has been common in the past. Instead, the continuous cylindrical mesh as-produced on a circular knitting machine is cut into lengths and placed as-is in the compression die.
The result has been a foraminous article that has been useful as a slag filter in automotive airbag assemblies. A slag filter is interposed in the flow path between the inflatable airbag on the one hand and, on the other hand, the explosive mixture that produces the gas used to inflate the airbag. The explosive mixture traditionally produces gas and molten metal called slag. The slag filter made from the compressed knitted wire mesh allows the inflating gas to pass freely but cools and traps molten metal.
When knitted wire mesh is cut, it results in the production of loose pieces of scrap commonly known in the wire knitting industry as tinkles. The material making up the tinkles had formally been a portion of the knit. In other words, a tinkle is a knitted loop, or a portion of a knitted loop, which has been cut. Tinkles are of irregular shape and distribution and have no predetermined location, size or shape. However, they do tend to remain near the cut line where they were formed.
In some contexts, tinkles create potential problems when they adhere to a continuous piece of wire mesh. One current method of removing tinkles is to try to shake them off the mesh sleeve or to pick them off by hand. Such methods are very labor-intensive, do not lend themselves to automation, and are subject to human error.
In the past, the present inventor successfully prevented the formation of loose tinkles in the fabrication of an aircraft wing tank gasket which required that a mounting hole be punched in the gasket. The gasket was produced from a spirally wound preform, which was then compressed to produce the overall substantially flat shape of the gasket. An electric resistance welding operation was conducted in the vicinity where the mounting hole was to be punched. This step successfully welded the adjacent plies of compressed mesh loops to each other in the vicinity of the weld. Any tinkles that were produced were bonded to the metallic structure and could not shake loose to do damage.
Such use of bonding has not heretofore carried over into the production of slag filters for automotive airbags. One possible reason is the need for great dimensional regularity in such filters. Carrying out a welding operation on such a filter might lead to a stiffer portion and resulting dimensional irregularity such as a raised band extending circumferentially of the filter near its opposite ends, where the tinkles congregate and where the bonding would be carried out.
So far as is known, before the current invention no one had considered the possibility of performing a welding or other bonding operation before compression. This would have been counter-intuitive, because the very act of compression might be thought to break the individual bonds. Moreover, when the fabrication of the product begins with continuous cylindrical mesh, as from a circular knitting machine, it is not apparent how one could weld the mesh in advance (before cutting) to prevent the initial formation of tinkles, as was done in the case of the hole to be punched in the airplane gasket. Therefore, so far as is known, tinkles have been present in airbag slag filters in the past, and have not been bonded to the filter.
As the design of automotive airbags has become increasingly critical over time, it has become even more necessary to provide the above-noted dimensional regularity in slag filters. Also, it has become increasingly desirable for the slag filters to be free of loose or potentially loose tinkles. Such tinkles might be captured in the final product by being mechanically held in the product. However, being unbonded, they have the potential to become free. It is thought that such tinkles can damage fragile airbag components during assembly and also during the life of the vehicle (as from vibration during use).
It is an object of the current invention to provide a method for the abatement of tinkles in a wire mesh product, and especially while maintaining high dimensional regularity of the product.
It is a further object of the current invention to provide a method for controlling tinkles in a product formed by compressing a single cylindrical ply of knitted wire mesh of the type produced on a circular knitting machine.
A still further object is to provide a product made from a compressed single ply of knitted wire mesh, being free of unbonded tinkles and being suitable for use as a slag filter as above-described.